1. Field of the Invention
The present invention relates to an image forming apparatus using an electronic photographic process such as a copier, printer or facsimile device, etc, and more particularly, to an image forming apparatus that comprises a charge apparatus that charges by superimposing an alternate current bias voltage on a direct current bias voltage as the charge bias voltage.
2. Description of the Background Art
The image forming apparatus in this type of electronic photographic system forms a toner image by: the charge apparatus discharging to the surface of a photosensitive member, which is the image support member; conducting charge processing by applying a charge of a specified polarity; forming an electrostatic latent image by exposing the surface of the charged photosensitive member; and the developing apparatus supplying to this electrostatic latent image toner that is charged in the same polarity as the charge polarity. The toner image formed on the photosensitive member is next transferred to recording paper, etc., and is fixed on the recording paper by heat and pressure.
Recent charge apparatuses have a charge roller, which is a conductive member formed in a roller-shape that contacts or nearly contacts the surface of a photosensitive member, and charges the surface of the photosensitive member by applying voltage between the charge roller and the photosensitive member when in proximity. These charge apparatuses have the advantages of low ozonization and low power consumption, and are often used as the method to charge the surface of the photosensitive member.
In order to conduct charge processing of the surface of the photosensitive member more uniformly, a system has been adopted in which a charge bias is applied that superimposes alternate current (AC) voltage on direct current (DC) voltage. Compared to DC only charge systems, the image forming apparatuses using this system must run a large AC current in order to obtain the desired charge potential, and it necessary to make the AC frequency at this time n times (n is an integer) or more the linear velocity of the photosensitive member. For example, if we let n=7, then with a photosensitive member linear speed of 100 mm/sec, 100×7, or 700 Hz or more is necessary. If not more than n times, then AC frequency jitters may be observed as halftone images in the uniform intermediate potential image. Consequently, if multiple photosensitive linear velocities are provided within one image forming apparatus, then the same kind of charge capacity can be obtained by making AC frequencies that match the various linear velocities.
However, it is necessary to provide a frequency conversion circuit in the power source unit of the charge apparatus when matching the linear velocity of the photosensitive member and varying the AC frequency, and this part raises the costs. Moreover, if fixed at the frequency necessary during the highest linear velocity, then jitters do not occur even during the lower linear velocities, but the excess discharge during low linear velocity is the cause of the adhesion, etc. of ozone-components, and produces filming on the surface of the photosensitive member. When filming forms on the surface of the photosensitive member, it becomes difficult to attenuate (lower) the superficial potential even if exposing to irradiated light, and the more the amount of attenuation is reduced the more laminated layers (thickness) of filming substance there are. As a result, the potential on the photosensitive member after exposure increases in conjunction with the increase of filming substance, specifically, the residual potential becomes notable and causes production of unsatisfactory images having scum, etc.
Thus, the following technology has been disclosed in order to eliminate filming.
For example, disclosed in Japanese Unexamined Patent Application Publication No. 2001-22225 and Japanese Unexamined Patent Application Publication No. 2000-147953 are technologies that detect filming on the photosensitive member using a surface potentiometer, and use a filming removal device to eliminate filming substance. Moreover, disclosed in Japanese Unexamined Patent Application Publication No. H2-191980 is a filming removal device that performs as a film removal action one cycle of filming removal when the main switch is thrown after processing a set number of pages.
Nonetheless, mounting surface potentiometers and filming removal devices increases the size of the configuration of the image forming apparatus, and raises the costs. Further, it is necessary to have a mechanism to prevent the occurrence of filming because it is extremely difficult to remove filming once films have formed on the photosensitive member.